In the purification of the exhaust gases of internal combustion engines, catalysts are frequently employed which include, i.a., components for storing the hydrocarbons contained in the exhaust gas.
Typical catalysts of this type are, for example, so-called diesel oxidation catalysts. They frequently include platinum-activated aluminum oxide or aluminum silicate as oxidation components and various zeolites as storage components. These catalysts are employed for the oxidative purification of the exhaust gases of diesel engines, i.e., they are designed to combust carbon monoxide contained in the exhaust gas and unburned hydrocarbons, that is to say, to convert them into carbon dioxide and water.
During engine operating phases at low exhaust-gas temperatures, the zeolites store the hydrocarbons contained in the exhaust gas. Conversion of the hydrocarbons in these operating phases is impossible as the catalyst's oxidation components are inactive due to the low temperatures during these operating phases. In modern diesel engines, such operating phases occur both after cold start and during idling phases and low-load operating phases, for example, in city traffic.
When the exhaust-gas temperature rises, the stored hydrocarbons are desorbed again and oxidized at the now active oxidation components. Examinations carried out by the inventors showed that when the temperature rises following prolonged operation of the catalyst at low temperatures and a correspondingly long storage period, sudden release and, at the same time, combustion of larger amounts of hydrocarbons can occur. As a result of this uncontrolled, vigorous combustion, the catalyst is exposed to high temperatures. This can result in damage to the catalyst. After desorption and combustion of the hydrocarbons, the storage components are ready to accept hydrocarbons again, provided no damage has occurred by possibly excessive temperatures during combustion.